In recent years, as an image pickup apparatus used for medial image diagnosis and nondestructive inspection, a radiation image pickup apparatus utilizing a flat panel detector (hereinafter referred to as an “FPD”) formed of semiconductor material has been put into practical use. Such a radiation image pickup apparatus is used as a digital image pickup apparatus for still-image shooting such as general shooting and moving-image shooting such as fluorography in medical image diagnosis.
Such a radiation image pickup apparatus includes the detector described above, a driving circuit which drives the detector, a reading circuit which reads an analog electric signal from the detector, and an A/D converter which converts the analog electric signal into a digital signal. The A/D converter outputs a digital image signal obtained at a time of shooting and an image signal used for correction. When an image signal is to be output from the image pickup apparatus within a short period of time, the image pickup apparatus includes a plurality of A/D converters.
However, the A/D converter may generate nonlinearity which is not ideal linearity characteristic as a conversion characteristic (A/D conversion characteristic) between an input analog electric signal and an output digital signal. In particular, if an image pickup apparatus has a plurality of A/D converters, nonlinearities of the A/D converters differ from one another and an image generated from digital signals may cause a feeling of strangeness due to unevenness of the image, for example. When such a feeling of strangeness is generated due to the unevenness or the like, it is preferable that the nonlinearity is suppressed or an adverse effect caused by the nonlinearity is corrected.
Patent Literature 1 discloses an A/D conversion circuit which stores a reference signal which is synchronized with a signal output from an A/D conversion unit as address data in a portion corresponding to an address specified by the signal output from the A/D conversion unit and which corrects the signal output from the A/D conversion unit in accordance with the reference signal. Use of this A/D conversion circuit enables reduction of a feeling of strangeness of an image caused by nonlinearity of the A/D conversion unit and realizes high image quality.
Furthermore, Patent Literature 2 discloses an A/D conversion circuit including a correction unit which corrects signals output from a plurality of A/D conversion units in accordance with one of the signals output from the A/D conversion units. Use of this A/D conversion circuit enables reduction of a feeling of strangeness caused by differences among nonlinearities of the A/D conversion units and realizes high image quality.